Tourmaline is a family of boron silicate minerals, and the black member — schorl — is the common one. It grows as long, striated prisms, and the striations are diagnostic: a genuine tourmaline bead usually shows fine parallel grooves running along the bead, which is not something glass reproduces.
Black tourmaline is often sold at prices that make natural material impossible, because opaque black stone is easy to fake with dyed quartzite. The check is the striation and the weight.